diff --git a/src/fallback.rs b/src/fallback.rs
index f49318c..92722af 100644
--- a/src/fallback.rs
+++ b/src/fallback.rs
@@ -1,6 +1,6 @@
 #[cfg(span_locations)]
 use std::cell::RefCell;
-#[cfg(procmacro2_semver_exempt)]
+#[cfg(span_locations)]
 use std::cmp;
 use std::fmt;
 use std::iter;
@@ -407,7 +407,12 @@
         })
     }
 
-    #[cfg(procmacro2_semver_exempt)]
+    #[cfg(not(span_locations))]
+    pub fn join(&self, _other: Span) -> Option<Span> {
+        Some(Span {})
+    }
+
+    #[cfg(span_locations)]
     pub fn join(&self, other: Span) -> Option<Span> {
         SOURCE_MAP.with(|cm| {
             let cm = cm.borrow();
diff --git a/src/lib.rs b/src/lib.rs
index 93e0523..0932131 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -420,8 +420,11 @@
     ///
     /// Returns `None` if `self` and `other` are from different files.
     ///
-    /// This method is semver exempt and not exposed by default.
-    #[cfg(procmacro2_semver_exempt)]
+    /// Warning: the underlying [`proc_macro::Span::join`] method is
+    /// nightly-only. When called from within a procedural macro not using a
+    /// nightly compiler, this method will always return `None`.
+    ///
+    /// [`proc_macro::Span::join`]: https://doc.rust-lang.org/proc_macro/struct.Span.html#method.join
     pub fn join(&self, other: Span) -> Option<Span> {
         self.inner.join(other.inner).map(Span::_new)
     }
diff --git a/src/wrapper.rs b/src/wrapper.rs
index 16a0849..af234ae 100644
--- a/src/wrapper.rs
+++ b/src/wrapper.rs
@@ -479,9 +479,9 @@
         }
     }
 
-    #[cfg(super_unstable)]
     pub fn join(&self, other: Span) -> Option<Span> {
         let ret = match (self, other) {
+            #[cfg(proc_macro_span)]
             (Span::Compiler(a), Span::Compiler(b)) => Span::Compiler(a.join(b)?),
             (Span::Fallback(a), Span::Fallback(b)) => Span::Fallback(a.join(b)?),
             _ => return None,